[PATCH] D28914: [AliasAnalysis] Fences do not modify constant memory location

[Anna Thomas via Phabricator via llvm-commits]

anna created this revision.

Fence instructions are currently marked as `ModRef` for all memory locations.

We can improve this for constant memory locations (such as constant globals),
since fence instructions cannot modify these locations.

This helps us to forward constant loads across fences (added test case in GVN).
There were no changes in behaviour for similar test cases in early-cse and licm.


https://reviews.llvm.org/D28914

Files:
  include/llvm/Analysis/AliasAnalysis.h
  lib/Analysis/AliasAnalysis.cpp
  test/Transforms/GVN/fence.ll


Index: test/Transforms/GVN/fence.ll
===================================================================
--- test/Transforms/GVN/fence.ll
+++ test/Transforms/GVN/fence.ll
@@ -1,5 +1,6 @@
 ; RUN: opt -S -basicaa -gvn < %s | FileCheck %s
 
+ at a = external constant i32
 ; We can value forward across the fence since we can (semantically) 
 ; reorder the following load before the fence.
 define i32 @test(i32* %addr.i) {
@@ -52,6 +53,25 @@
   ret i32 %res
 }
 
+; We can forward the value forward the load
+; across both the fences, because the load is from
+; a constant memory location.
+define i32 @test4(i32* %addr) {
+; CHECK-LABEL: @test4
+; CHECK-NOT: load
+; CHECK: fence release
+; CHECK: store
+; CHECK: fence seq_cst
+; CHECK: ret i32 0
+  %var = load i32, i32* @a
+  fence release
+  store i32 42, i32* %addr, align 8
+  fence seq_cst
+  %var2 = load i32, i32* @a
+  %var3 = sub i32 %var, %var2
+  ret i32 %var3
+}
+
 ; Another example of why forwarding across an acquire fence is problematic
 ; can be seen in a normal locking operation.  Say we had:
 ; *p = 5; unlock(l); lock(l); use(p);
Index: lib/Analysis/AliasAnalysis.cpp
===================================================================
--- lib/Analysis/AliasAnalysis.cpp
+++ lib/Analysis/AliasAnalysis.cpp
@@ -367,6 +367,14 @@
   return MRI_Mod;
 }
 
+ModRefInfo AAResults::getModRefInfo(const FenceInst *S, const MemoryLocation &Loc) {
+  // If we know that the location is a constant memory location, the fence
+  // cannot modify this location.
+  if (Loc.Ptr && pointsToConstantMemory(Loc))
+    return MRI_Ref;
+  return MRI_ModRef;
+}
+
 ModRefInfo AAResults::getModRefInfo(const VAArgInst *V,
                                     const MemoryLocation &Loc) {
 
Index: include/llvm/Analysis/AliasAnalysis.h
===================================================================
--- include/llvm/Analysis/AliasAnalysis.h
+++ include/llvm/Analysis/AliasAnalysis.h
@@ -443,11 +443,7 @@
 
   /// getModRefInfo (for fences) - Return information about whether
   /// a particular store modifies or reads the specified memory location.
-  ModRefInfo getModRefInfo(const FenceInst *S, const MemoryLocation &Loc) {
-    // Conservatively correct.  (We could possibly be a bit smarter if
-    // Loc is a alloca that doesn't escape.)
-    return MRI_ModRef;
-  }
+  ModRefInfo getModRefInfo(const FenceInst *S, const MemoryLocation &Loc);
 
   /// getModRefInfo (for fences) - A convenience wrapper.
   ModRefInfo getModRefInfo(const FenceInst *S, const Value *P, uint64_t Size) {


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